Revealing the multiple cathodic and anodic involved charge storage mechanism in a FeSe2 cathode for aluminium-ion batteries by in situ magnetometry

Rechargeable aluminium-ion batteries (AIBs) are considered to be promising alternatives for current lithium-ion batteries (LIBs), since they can offer the possibilities of low cost with high energy-to-price ratios. Unlike in LIBs, the charge storage mechanism in AIBs involving different ionic species is far more complicated and remains largely unexplored, which impedes further screening and optimization of cathodes materials that can reversibly accommodate aluminium-based (complex) charge carriers with boosted cell performance. Here, we report a comprehensive study of the battery chemistry in metal selenide based cathode in AIBs from an integrated chemical and physical point of view. Various in situ and ex situ characterization techniques and theoretical calculations reveal that both Cl- and AlCl4- can act as the charge carriers in the FeSe2 cathode during the charge process, and the Al3+ can also be embedded into the host upon discharge process. Furthermore, using in situ magnetometry, the spin-polarized surface capacitance is observed in AIBs for the first time, which proves that Al3+ can serve as charge compensation in the formation of space charge zones with electrons. These innovative findings provide unprecedented insight into the charge storage mechanism of AIBs.